Relaying is considered for LTE Advanced as a tool to improve, for example, the coverage of high data rates for User Equipment (UE), temporary network deployment, cell edge throughput and/or to provide coverage in new cell areas. LTE Advanced supports relaying by having a Relay Node (RN) wirelessly connected to a base station (eNB) (referred to as a Donor eNB (DeNB)). In addition to serving its own ‘donor’ cell, the Donor eNB serves the RN, via a modified version of the Evolved Universal Terrestrial Radio Access (E-UTRA) radio interface. The modified interface is referred to as the ‘Un’ interface.
Each RN is provided with many aspects of a base station's functionality and is therefore capable of acting as a base station serving its own ‘relay’ cell. From the perspective of the user equipment (such as mobile telephones) in the relay cell, therefore, the RN essentially appears to be a conventional LTE base station. In addition to the base station functionality, however, the RN also supports a subset of the UE functionality including, for example, many aspects of the physical layer, Medium Access Control (MAC), radio resource control (RRC), and non access stratum (NAS) functionality, to allow it to connect wirelessly to a Donor eNB. From the perspective of the Donor eNB, therefore, the RN essentially appears to be an item of user equipment such as a mobile (cellular) telephone.
As mobile telephones move around in the area covered by the communication system, they are handed over from one cell (i.e. base station) to another, depending on signal conditions and other requirements, such as requested quality of service, the type of service used, overall system load, and the like. A trigger for handing over a mobile telephone to a new cell may be based on measurements of the neighbour cells performed by the particular mobile telephone. The type of triggers and the related measurements to be performed by mobile telephones are detailed in section 5.5.4 of the 3GPP TS 36.331 v10.5.0 standard. In particular, the above standard defines measurement report triggering related to eight different event types (Events A1 to A6, B1, and B2) that the base station may configure for user equipment within its cell(s). In summary, such triggers may generally relate to an event when the mobile telephone's serving cell (or a neighbouring cell) becomes better (or becomes worse) than either a pre-defined threshold or a pre-determined offset value.
Further details of the overall mobility sequence are described in section 10.1.2 of the 3GPP TS 36.300 standard, which describes the configuration of measurements by the base station and the subsequent triggering of handover.